This invention relates to a system and/or method for interlocking a power supply in, for example, a printing apparatus such as a copy machine or printer, to prevent user or technician exposure to potentially hazardous direct current (DC) voltages or mechanically hazardous systems which derive their power from such DC voltages. The system is arranged so that an interlock switch connected to the housekeeping power supply of the apparatus and a power transformer, which only transmits alternating current (AC) from its primary side to its secondary side, are used to terminate power to areas of the apparatus which are potentially hazardous to users or technicians. Opening the interlock switch, upon the occurrence of an event such as opening of a copy machine door, ultimately prevents transmission of AC power through the power transformer. Thus, no AC potential is developed on the secondary side of the transformer and, consequently, no DC voltage is developed at the output of the system.
While the invention is particularly directed to the art of interlock switching, and will thus be described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications where it is desirable to interrupt large amounts of DC power in a fail-safe manner, using only low power switching devices.
International safety organizations typically require that consumer products be designed to protect consumers from any potential hazards of the product, especially electrical hazards. With respect to copy machines and printers, voltages of varying magnitude, corresponding to various parts of each machine, are present in each machine. Certain of these voltages, and related current levels, exceed that which is considered safe. Thus, potential hazards exist for users and technical personnel who maintain and service such machines.
Accordingly, either 1) restricting access to hazardous areas of the machine to only technical personnel, and/or 2) removing potential hazards when an operator gains access to a hazardous area of the machine, have become accepted preventative techniques.
Regarding the former technique, potentially hazardous areas are oftentimes enclosed with a protective insulated cover and secured to prevent access by unauthorized persons. As to the latter technique, the removal of potential hazards has traditionally been accomplished by disconnecting DC power to the source of the potential hazard. Since the systems are unable to develop any hazardous conditions without power, this has proven to be successful. In copy machines, an interlock switch is connected to the door of the machine so that when the door is closed, the switch is closed. Likewise, when the door is opened, the switch is opened to terminate the supply of power creating the potential hazard.
Power is typically disconnected by connecting the interlock switch in such a manner so as to directly interrupt DC power or by operating a power relay, with an interlock switch in the relay coil circuit, which interrupts DC power to the potentially hazardous area or system. FIG. 1 shows an example of the power relay interruption system. More particularly, power supply A, a 24 volt switch mode power supply (SMPS) having a 12-15 volt housekeeping supply included therein, is connected to interlock relay C having interlock switch B in its relay coil circuit. The switch B is in a normally closed position. Upon detection of an undesirable condition, i.e., opening of a door creating the potential for a hazardous condition, the switch B opens. Thus, the relay C is de-energized and power to the output is interrupted.
These traditional systems, however, involve switching high current loads in high stress conditions since the output current load is oftentimes in the range of 15-50 amps. As a result, undesirable noise transients are developed, arcing occurs, switch wear develops, and disturbances are induced on the DC power lines.
The present invention cures the above noted deficiencies and provides an improved interlock switching system.